1. Field of the Invention
The present invention relates to a method for controlling the displacement or capacity of a wobble plate type compressor with a wobble angle control unit for adjustably changing an angle of the wobble plate in the crankcase of the compressor by changing a fluid pressure level within the crankcase, and more particularly, relates to a method of controlling the operation of a solenoid-operated fluid-flow control valve of the wobble angle control unit, which valve is operated by an electric current supplied in the form of pulses the duty ratio of which is changed in response to a change in the requirement for compressor displacement.
2. Description of the Related Art
Many variable displacement wobble plate type compressors with a wobble angle control unit are known. For example, U.S. Pat. No. 4,685,866 to Takenaka et al discloses a variable displacement wobble plate type compressor having a crankcase for receiving a variable angle wobble plate mechanism for driving the reciprocatory movement of pistons and a wobble angle control unit accommodated in a housing for defining a suction chamber for refrigerant before compression and a discharge chamber for refrigerant after compression. The wobble angle control unit includes a fluid flow control valve with which a fluid pressure level within the crankcase is changed, thereby causing a change in a wobble angle of the wobble plate. The fluid flow control valve of U.S. Pat. No. 4,685,866 is operated by a pressure-sensitive movable valve mechanism.
A pending U.S. patent application Ser. No. 093,797, filed on Sept. 4, 1987 discloses a variable displacement wobble plate type compressor with a solenoid-operated wobble angle control unit for adjustably changing the angularity of the wobble plate, to thereby control the compressor displacement.
FIG. 1 illustrates the general mechanical construction of the variable displacement wobble plate type compressor of the pending U.S. patent application Ser. No. 093,797. The compressor 1 has a cylinder block 2 having cylinder bores in which pistons 3 are reciprocated, a crankcase 4 having therein a chamber 5 for receiving a wobble plate assembly including a rotatable drive plate 9a supported on a drive shaft 9c and a non-rotary variable angle wobble plate 9b for driving the reciprocating movement of the pistons 3, a housing 6 having therein a suction chamber 7 and a discharge chamber 8, and a solenoid-operated wobble angle control unit including a solenoid-operated valve means 10 accommodated in the housing 6. The solenoid valve means 10 of the solenoid-operated wobble angle control unit is arranged for control of the fluid communication between the crankcase chamber 5 and the discharge chamber 8 by means of a fluid passageway 11 in response to a change in the cooling load of the compressor. The solenoid valve means 10 has a solenoid element which can be energized and de-energized in response to an external control signal. A fluid passageway 12 is arranged for communicating the crankcase chamber 5 and the suction chamber 7 of the housing 6 as required.
The solenoid-operated valve means 10 of the solenoid-operated wobble angle control unit changes a fluid pressure level Pc within the chamber 5 of the crankcase 4 which acts on the back faces of the pistons 3, to thereby cause a change of a pressure differential (Pc-Ps) between the fluid pressure Pc and the pressure Ps in the suction chamber 7. When the pressure differential (Pc-Ps) is larger than a predetermined pressure level, the stroke of each piston 3 is decreased to vary the angularity of the wobble plate 9b toward an erected position from an inclined position. Thus, the compressor displacement is decreased. On the other hand, when the pressure differential (Pc-Ps) is much smaller than the predetermined pressure level, the stroke of each piston 3 is increased to move the wobble plate 9b toward the inclined position, and the compressor displacement is increased. At this stage, as stated above, the operation of the solenoid-operated valve means 10 of the solenoid-operated wobble angle control unit is controlled by energizing and de-energizing the solenoid element thereof. However, the conventional method of energizing and de-energizing the solenoid-operated valve means 10 is conducted by changing the duty ratio .delta. of an electric excitation current supplied to the solenoid element in the form of pulses having a predetermined frequency value (f) selected from the range from two to ten or more hertz (Hz). The duty ratio .delta. is defined by the following equation, i.e., .delta.=t/T, where t indicates the time period in which the electric excitation current of each pulse appears, and corresponds to the width of the electric pulsive current, and T indicates each time cycle of the electric pulsive current and is equal to 1/f. Therefore, changing the duty ratio of the pulsive electric excitation current means changing the time period t in which the electric excitation current of each pulse appears. However, with the above-mentioned conventional method, changing the duty ratio .delta. of the electric pulsive excitation current will not cause an immediate change in the angularity of the wobble plate 9b, i e., an immediate change in the compressor displacement. Thus, the present inventors carried out research and conducted experiments to determine how to overcome the inability of the conventional method to control the compressor displacement of a variable displacement wobble plate type compressor by controlling the duty ratio on the solenoid-operated valve means 10. As a result, it was discovered that, when a control of the duty ratio of the electric pulsive excitation current for the solenoid-operated valve 10 is carried out so as to adjustably change the pressure level Pc in the chamber 5 of the crankcase 4, to thereby change the angularity of the wobble plate 9b, a frictional force generated between the wobble plate 9b and the associated various mechanical elements prevents an immediate movement of the wobble plate 9b in response to a change in the pressure level Pc in the crankcase chamber 5. Namely, since the frequency (f) of the electric pulsive current for energizing the solenoid operated valve means 10 is from two to ten or more hertz, which is rather high, as will be understood from the illustration of FIG. 6, the time cycle (T) of the electric pulsive current is small. Therefore, as illustrated in FIG. 7, when the duty ratio is maintained at a value between 0 to 1.0, the pressure level Pc within the crankcase chamber 5 is maintained at a very stable level in the form of an extremely small wave. Accordingly, even if the duty ratio .delta. of the electric pulsive current having the small time cycle (T) is slightly changed, an appreciable change in the pressure level (Pc) within the crankcase chamber 5, which can change the angularity of the wobble plate 9b by overcoming the friction force, does not occur. That is, when the angularity of the wobble plate 9b is changed from an inclined position to an erect position, and vice versa, the pressure level (Pc) within the crankcase chamber 5 must come out of a specified pressure range between the two high and low pressure levels Pc1 and Pc2 which are defined as the pressure levels necessary for commencing the erecting and inclining movement of the wobble plate 9 against a friction force. However, the slight increase or decrease alone of the duty ratio .delta. of the electric pulsive current cannot bring about a sufficient change in the pressure level Pc over the pressure level Pc1 or below the pressure level Pc2. Accordingly, although the duty ratio .delta. per se might be successively changed, the angularity of the wobble plate 9b cannot be successively changed in a direct relationship with the change of the duty ratio .delta.. Consequently, when it is necessary to change the angularity of the wobble plate to control the compressor displacement, the duty ratio .delta. must be unduly increased or decreased until the pressure level Pc prevailing in the crankcase chamber 5 is appropriate for commencing a movement of the wobble plate 9b while overcoming the above-mentioned friction force. At this stage, however, due to this undue change of the duty ratio, the movement of the wobble plate 9b for changing the angularity thereof always results in an overshoot of a desired angular position of the wobble plate 9b. Therefore, conventionally it takes rather long time to control the compressor displacement by changing the duty ratio of the electric pulsive energizing current of the solenoid-operated valve means 10 of the solenoid-operated wobble angle control unit.